Vista ultimate 64 bit and 5 gigs of Ram

My motherboard Asus p5w dh deluxe has three slots for ram, i have 2 slots with 2gig each and 3rd with 1 gig all matching Ram. I am supposed to be able to use up to 8 gigs with Vista ultimate 64 bit but my computer only sees 3199mb as stated on system properties page. Did I waist money or can someone tell me how to utlize all 5 gigs?

Thank you for your help in advance. I know how to access my bios info but im no expert. If explained in fairly simple terms I can make most adjustments.

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